Field of the Invention
The present invention relates to a device for converting and coupling outputs representative of process variables from transmitters to a process controller.
In a process control system, a process controller monitors a process by communicating messages with field sensing devices. The field devices are of various types and may communicate with different communications standards. The process controller may communicate with yet another communications standard. When the process controller and a field device type communicate with different standards, a "communicator" provides a translating interface between them.
A communication standard governs both composition and encoding of messages and has a messaging layer and a physical layer. The messaging layer defines a set of rules for combining data structures into a message. A data structure is a grouping of digital bits having a specific meaning. For example, a data structure meaning "read a floating point register" may be defined as binary 01100101. The physical layer, on the other hand, defines a set of rules for encoding the digital bits onto a physical medium such as an interface. The rules specify the AC and DC parameters for encoding each bit, the number of conductors used to encode the bit and associated timing. For example, one physical layer encodes a logic "0" as 0.0 Volts on a wire in an interface. RS-232, RS-422 and RS-485, defined by the Electronics Industry Association, are examples of IEEE serial interface physical layers used by the process control industry.
Process controllers typically communicate in a "MODBUS" messaging layer encoded on an IEEE serial interface layer. The "MODBUS" communications standard is defined in GOULD "MODBUS" PROTOCOL document P1-MUS-300, Rev. B. Field devices use either a "MODBUS" message layer or a Mark-Space message layer, both encoded on a Mark-Space physical layer. The Mark-Space communications standard is used in Varec Division of Emerson Electric products.
Mark-Space communications require two conductors to encode the information while a third conductor is a ground reference. An active low pulse on the first conductor encodes a binary one and an active low pulse on the second conductor encodes a binary zero. There is an off state between any two pulses.
Existing communicators convert and couple messages between a process controller and a single type of field device. In some cases, multiple communicators are needed to couple messages between a process controller and one type of field device. Consequently, at least two types of communicators are interposed between the process controller and two field device types. In the simplest case, one type of communicator interposes between the process controller to field devices messaging in "MODBUS" and a second type interpose between the same process controller and field devices messaging in Mark-Space. Each communicator type requires a separate network of interconnections, impacting cost and connection complexity.
Communicators include a memory loaded with pairs of corresponding "MODBUS" addresses and field device addresses, where the "MODBUS" addresses point to memory locations containing process variables in the corresponding field device. Typically the loading, or configuration of the memory is performed when the process control application is started or modified. These memory contents are loaded via commands from the process controller using special purpose, expensive configuration software.
Furthermore, existing communicators extract commands from incoming messages and validate the command by matching the extracted command to a stored set of commands. Consequently, when the process controller is upgraded to send new commands, the communicator must be redesigned.
Consequently, there is a need for a communicator interposed between both types of field devices and a process controller to minimize connection complexity and cost, yet which is able to support command set upgrades without redesign and easily configured at start-up and modification.